This invention pertains to an aqueous inkjet ink, in particular to an aqueous inkjet ink comprising an aqueous vehicle, a pigment colorant and a water-soluble silver salt.
Inkjet recording is a printing method wherein droplets of ink are ejected through fine nozzles to form letters or figures on the surface of recording media. Inks used in such recording are subject to rigorous demands including, for example, good dispersion stability, ejection stability, and good fixation to media.
Aqueous dispersions of pigment particles are widely used in ink-jet printing. Because a pigment is typically not soluble in an aqueous vehicle, it is often required to use a dispersing agent, such as a polymeric dispersant or a surfactant, to produce a stable dispersion of the pigment in the aqueous vehicle. More recently, so-called “self-dispersible” or “self-dispersing” pigments (hereafter “SDP(s)”) have been developed. As the name would imply, SDPs are dispersible in water without dispersants.
The main advantage of inkjet printers over laser printers includes low cost and simplicity. However, the optical density of ink jet inks on plain paper is lower than the optical density that can be obtained by laser printers. In addition, the time duration for pigment ink to crust in the nozzles of the pen (“decap time”), a parameter required to improve optical density, is too short, especially for those inks with high levels of pigment, consequently resulting in the needs for frequent maintenance leading to lower printer throughput and wasting of ink.
U.S. Pat. No. 6,332,919 discloses black inkjet inks made with SDP and an added monovalent salt to improve optical density. The monovalent salt is an alkali metal, ammonium or organo-ammonium.
U.S. Pat. No. 6,277,183 discloses a black inkjet ink comprising an SDP ink and a metal oxide such as silica or alumina to improve optical density.
U.S. Pat. No. 6,899,754 discloses aqueous inkjet inks containing self-dispersing pigments and multivalent cations to improve optical density.
A need exists for an inkjet ink that has higher optical density on plain paper with improved decap performance. In particular, there is a need for a black ink with higher optical density, improved decap performance and good long-term stability. The present invention satisfies this need by providing compositions having higher optical density, improved decap performance and good long-term stability.